Extract path configurations for Revit System Tests

Background
==========

Revit system tests are dependent on file paths hardcoded in individual tests. This doesn't allow us to revit tests from any arbitrary location. We need a very specific rigid folder structure of support files to run the revit tests.

This submission aims to extract the path related info to a configuration file and in presence of RevitTestConfigurations.xml all the paths are read from there and test data is resolved from this path otherwise it will be resolved from default path location.

*Reviewers*
@Randy-Ma 
SetDefaultValue() sets value to properties which are null or empty. we may not have all properties defined in xml file, so rest of the properties will be set as default values.